When radiant energy from the sun strikes a solar collector, some of the energy is reflected or transmitted and lost, and the remainder is either absorbed or re-radiated into the atmosphere. Most of the sun's energy is emitted at wavelengths below 2.0 microns and a substantial amount of such energy is normally re-radiated into the atmosphere at a longer wavelength, such as infrared radiation. It is desirable, therefore, for solar collectors to have a high capacity for energy absorption below 2 microns in combination with low emissivity at the longer wavelengths.
Blackbodies are known to absorb a significant amount of energy in the solar spectrum but, unfortunately, also re-radiate most of that energy in the infrared (IR) spectrum and, therefore, are generally unsatisfactory as collectors.
Preferred absorptive coatings or films, which optimize solar absorptance and inhibit emissivity in the longer wavelengths, are generally known as "selective absorbers". A superior selective absorber should have, for instance, a solar absorptance level (.alpha.) approaching a value of 1.0 and a thermal emittance or emissivity (.xi.) approaching zero.
The principal factors affecting absorptance, emittance and thermal stability of solar energy collectors are (1) the physical and chemical properties of the absorptive layer, (2) the nature of the substrate to which they are applied, and (3) the nature and functional properties of the precoat or interlayer between the absorptive component and substrate.
Metals such as silver, copper, gold and aluminum, for instance, have low emissivity but are low absorbers of solar energy and, alone, are not generally useful as solar collectors. They can, however, be incorporated into a selective solar absorber collector as a low emissivity component. In addition, some systems have been found unstable, particularly under vacuum conditions. As a result, there is a rapid deterioration of emissivity properties, and substantial diminishment in the overall efficiency of the collector. At higher temperatures, by way of example, silver films tend to agglomerate. In addition, metal oxide components tend to break down under vacuum to produce the elemental form.